Aluminum heat exchangers are used widely in the automotive industry. Applications include engine cooling systems where heat exchangers such as radiators, oil coolers, charge air coolers and the like are employed. Additionally, passenger climate control systems also utilize heat exchangers as evaporators and condensers. Air conditioning systems typically have two heat exchangers: condensers and evaporators. Condensers typically operate at elevated pressures (>500 psi) and most typically are made with extruded tubes. These tubes are produced via traditional methods of extrusion. Most radiators use tubes made from welded or folded sheet (in part, due to the elevated costs per unit length of extruded tubes versus tubes made from sheet and in part as a result of the lower system pressure requirements of radiators (typically 20 psi or less)). However, some radiator tubes are also manufactured by extrusion processes depending on the design and specific durability requirements of the radiator in the field.
Typical extrusion based heat exchangers come essentially in two designs. The first design uses round tubing and bare (i.e. uncladed) fins that are mechanically attached to the round tubes by first lacing the tubes into holes punched in the fins, and then expanding the tubes to ensure that the tube's outer surface is in close mechanical contact with the fins.
The second typical design uses flat tubing having a plurality of channels in the tubing, commonly referred to as multi void tubing or micro multi void tubing. This type of heat exchanger tubing is attached to the fins using a brazing process. The cross section of the flow channels can vary, e.g., circular, oval, square, rectangular, of other regular or irregular shapes. Typically, micro multi void and multi void tubing are about 10-60 mm in width and about 1-2 mm in height.
Most aluminum extruded tubing products made for heat exchanger systems are produced by using traditional billet extrusion methods, such as press extrusion. As the alloy strength increases, the ability to use these traditional methods of extrusion becomes less feasible due to the difficulty of extruding the high strength alloy into a small tube. This difficulty occurs because as the dimensions of the heat exchanger tube decreases, the extrusion ratio, which is defined by a ratio of container bore area and the total cross sectional area of extrusion, increases thereby increasing the extrusion pressure needed to extrude the heat exchanger tube. The end result is that it is difficult (or sometimes not possible) and at the least, very expensive to produce fine dimensioned tubing from high strength materials using traditional extrusion methods. This is becoming a barrier to the introduction of newer heat exchanger designs that utilize fine dimensioned tubing, particularly for next generation designs where tubing must withstand very high pressures, for example CO2 refrigerant systems. In addition, as the heat exchanger manufacturers continue to work on next generation designs there is an increasing need for thin walled tubing with finer features that has high strength and high corrosion resistance, and in some instance high strength at elevated service temperature. Therefore, heat exchanger tubing might soon be required to operate at significantly higher pressures while at the same time becoming smaller in size as compared to current heat exchanger designs with tubes that use R-134a as the refrigerant. In order to meet these increasing demands in a cost effective manner, it is critical that the heat exchanger tubes be extruded from higher strength alloys.
Therefore, there exists a need for an improved high strength heat exchanger tube that can withstand high burst pressures, exhibit good corrosion resistance, have small dimensions (e.g. 10 mm by 1 mm MMV tubing with micro void width less than 1 mm), and have fine internal features. Additionally, there exists a need for a process to economically fabricate such a tube.
A continuous rotary extrusion process, known as the Conform™ Process, was developed and patented (U.S. Pat. No. 3,765,216) by the United Kingdom Atomic Energy Authority. In the Conform™ Process a rod or particulate feedstock replaces the extrusion billet and thereby making the extrusion process continuous. The equipment used in the Conform™ Process includes a grooved wheel, a coining roll, an abutment, a close fitting shoe, and an extrusion die. During the extrusion process the feedstock is fed into a space formed by the grooved wheel, close fitting shoe, and abutment, and is heated and pressurized by the friction between the rotating wheel and metal. When the metal temperature is sufficiently high the pressure extrudes the metal through the extrusion die.
For high volume aluminum tubing production requiring high productivity (lbs/hr), capital investment generally has not particularly favored the Conform™ process (vs. the traditional billet extrusion process) because of developed multi-out capabilities of billet extruders (sometimes up to 8 out) and differences in market pricing for feedstock rod vs. billet (i.e. billet is generally cheaper per pound for a specific alloy). Furthermore, it is a general perception in the industry that the Conform™ process is more appropriate for simple, larger tolerance, less demanding shapes made from easy to extrude alloys like AA1060 for products such as spacer bars and the like. Hence the overwhelming majority of tubing is produced today via traditional billet based extrusion processes.
However, the Conform™ process can make certain shapes with certain metallurgical structures that billet based process cannot make due to the process differences between the two processes, namely due to the differences in metallurgical structure of the rodstock and the reduced extrusion ratios (and resulting constancy of the die face pressure) of the Conform™ process.
Thus, there is a problem in the art, associated with the difficulty in extruding a small tube having a relatively complex structure, such as a micro multi void tubing, from a material with a high flow stress because the small tubing size increases the extrusion ratio, i.e. the cross sectional area ratio between the billet and the extruded product. This in turn increases the tonnage of pressure needed to extrude the alloy into the desired shape and dimension, and effectively limits the alloy chemistries that can be chosen as well as the allowable base microstructure of the metal being extruded to microstructures that have low flow stresses. Hence billets are generally homogenized prior to extrusion, solute contents are generally low, and the billets are generally DC cast and preheated immediately prior to extrusion process.